This invention relates to a terminal device for CATV which receives multichannel TV signals and a downstream data signal both sent from a head end, converts the frequency of any one of the TV signals into a value adapted to be received in an ordinary TV receiver, and if it is required, executes the instruction included in the downstream data signal.
Various types of pay television and CATV systems are known. In one type of system, unmodified TV signals are sent unidirectionally from a central station (or head end) to a terminal device. In another type of system, not only TV signals but also a control signal (downstream data signal) are transmitted to control the operation of the terminal device and furthermore actuate indicators and other devices. In still another type of CATV system, in addition to the above described TV signals and control signal sent from the head end to the terminal device, another signal (upstream data signal) is transmitted for reporting the operational condition of the terminal device to the head end, thus constituting a bidirectional transmission system.
In the types of the systems wherein not only TV signals but also downstream data signals are transmitted, the terminal device should have functions for converting the frequency of any one of the TV signals to a value receivable by a conventional TV receiver, and also for receiving and analyzing the downstream data signal and performing a predetermined operation. To this end, it is required that the received signal over a cable be separated into TV signals and the downstream data signal, and that a buffer circuit be provided for eliminating deleterious effects between the separating circuits of the TV signals and the downstream data signal, and for preventing interference between the separating circuit and other circuits connected prior or subsequent to the separating circuit. For example, the CATV signal bandwidth used by the head end system may encompass 50-300 mHz with the data signal having a carrier frequency of about 115-120 mHz and the adjacent video channel being separated only by a few megahertz. Under these conditions, extremely sharp notch and bandpass filter techniques must be used, and even so the adjacent video channel can be adversely affected by the nonuniform characteristic of the edge of the passband of the filter.
A primary object of the present invention is to provide a terminal device for CATV, wherein any adverse effect on the TV signal from the downstream data receiving portion is prevented.
Another object of the invention is to provide a terminal device for CATV, wherein no specific provision of buffer circuit is required for preventing deleterious effects caused by the receiving portion of the downstream data against the TV signals, and the circuit configuration thereof is much simplified.